Can't enter debugging mode

I checked the debugging option under Applications>Development, then powered off. Pressed and held Power and Volume Down buttons. I get the Samsung Galaxy Tab 10.1 screen, but instead of the two android icons immediately afterward, the screen goes black, then the Tab 10.1 screen again, followed by screen going black again. This repeats for as long as I hold the two buttons. When I release, it boots normally.

I had a 10.1 Tab before and had no problems getting into debugging mode.